Abstract: The predominant brevetoxin produced by Karenia Brevis, PbTx-2 was added to 0.2 µmfiltered
Wrightsville Beach, North Carolina seawater (WBSW) and irradiated under simulated
sunlight for six hours to determine its photolability. The average first order rate constant of PbTx-2
degradation was 0.20 ± 0.05 h-1 at 25 ºC. When PbTx-2 was added to UV-irradiated WBSW, the
average rate constant decreased significantly to 0.08 ± 0.03 h-1 at 25 ºC relative to that of untreated
WBSW suggesting dissolved organics are involved in the photodegradation of PbTx-2 in seawater.
UV-irradiated WBSW treated with Chelex-100 resin to remove trace metals caused no degradation
of PbTx-2 upon exposure to sunlight suggesting that trace metals, in addition to organics, are
involved in the photodegradation of PbTx-2. Irradiation of PbTx-2 in deoxygenated filtered
WBSW caused the toxin to degrade almost immediately upon exposure to simulated sunlight. The
effect of deoxygenation on the photodegradation of other brevetoxins including PbTx-1, 3, 6 agreed
with the rate of PbTx-2. PbTx-9 and the naturally-produced compound, brevenal, degraded
considerably in deoxygenated WBSW but not as rapidly. These results demonstrate that
photosensitizers are involved in the degradation mechanism, suggesting that PbTx-2 degrades by
secondary photochemical pathway. The mechanism suggested here is that PbTx-2 degrades in the
presence of the excited chromophoric dissolved organic matter (CDOM)* and trace metal complex.
Furthermore, the removal of oxygen from seawater causes the rate of degradation to increase
rapidly because (CDOM)* no longer scavenges oxygen thereby scavenging solely PbTx-2. A
model is also proposed which suggests that 75% of PbTx-2 in coastal seawater can be removed in 6
hours by exposure to sunlight. This represents a significant impact on the residence time of the
toxin in bloom indicating photodegradation is an important and perhaps the dominant sink of the
toxin in coastal seawater.